Online is typically faster. But not by much. My preference is to mail petitions in so I can make sure everything is there. Don't staple or tab anything as that does slow down processing by the contractor.
I'd find it hard to believe that he doesn't have at least enough interest or passive income that wouldn't trigger a tax return. But if he didn't need to file then at least a letter from him to explain that will be needed. Bank statements will be used to show the sponsorship ability.
There are no benefits of taxes in filing for a CR-1. The "advantage" may apply once you are married and you can file jointly. As previously mentioned the spouse would need a ITIN.
LOL it is literally taking the worst things about a K1 and Mixing it with a CR-1. It is free so go for it. You know how many have been issued in the last fiscal year?